problems with find

I don't know the fundamental thing that I'm misunderstanding here about
the find().

In my action "view_style" i want to list the sizes related to the the
style (which is the id I'm looking at).

My tables are sizes, styles and products, where products is a join
table with id, size_id and style_id, so that size\ "has_many :styles,
:through => products" and style "has_many :sizes, :through => products"

So i need to write something along the functionality of (this

  @sizes = find ( size_ids where style_id == params[:id])

in order that i can loop through the sizes in a select box in the view,
but for whatever reason, everything that I've tried has totally failed.

I'm at a loss and I've been stalled on this problem for a while now.
Can anyone help?